1. Field of the Invention
The present invention relates to an image synthesizer and, more particularly, to an image synthesizer which is capable of synthesizing and outputting an image signal at a real time on the basis of image information which is outputted from an image information supply source.
2. Description of the Prior Art
An image synthesizer synthesizes and outputs various image signals for CRT display on the basis of image information supplied from the outside. Since it is able to synthesize and output not only a two-dimensional plane image signal but also a signal for the two-dimensional image of a solid body, namely, a pseudo-three-dimensional image signal, it has been finding wide applications in, for example, three-dimensional video games, simulators for airplanes and various other vehicles, computer graphics and displays of CAD apparatus.
Such an image synthesizer conventionally adopts what is called a bit-map-display (graphic display) technique, and is therefore provided with a bit-map-memory which has memory areas having one-to-one correspondence with respect to all the pixels on the display of the CRT.
Into each of the memory areas are written all pieces of pixel information to be displayed in one frame. For example, when a given figure is displayed in computer graphics or the like, the outline of the figure is written on the CRT and the inside of the figure is colored out by a designated color which has been written in the memory.
In such an image synthesizer it is often the case that a plurality of figures are to be displayed simultaneously. Particularly, when a plurality of figures are displayed with overlapping portions, there is a problem regarding how the overlapping portions are to be colored.
A device which colors figures in the order of priority and a device which colors figures in the reverse order of priority are conventionally known as devices for dealing with such a problem.
Both of these conventional devices, however, have the following problems (A) to (C), and effective means for solving these problems have been in demand.
(A) The former type of device suffers from the problem that it is difficult to obtain a real-time display of a rapidly moving image, because it is very difficult for it to process an image quickly.
When a plurality of figures are successively colored in the order of priority, namely, from a figure having a high priority (situated in the foreground) to a figure having a low priority (situated in the distance), and they are displayed with overlapping portions, it is necessary to prevent the data on the figure having a higher priority, which is written first, from being deleted by the data on the figure which is to be colored later.
Therefore, in such a conventional device, all data on the figures to be colored out are read from the respective data write areas prior to the coloring out process, and judgement is made as to whether or not the data are written in each area. The coloring out process is only executed in the areas which have been judged to have no written data. This operation is called a read-modify-write operation.
Consequently, high-speed coloring out processing with respect to the bit-map-memory is impossible in the conventional device, and the coloring out operation of the bit-map-memory is often unable to catch up with a change in images. In particular, it is impossible to display a rapidly moving image at the real time.
It is possible to execute the read-modify-write operation with high speed in the conventional device by increasing the capacity of the data bus used.
However, increase in the capacity of the data bus leads to an increase in size and a rise in the cost of the entire device relative to the amount of information to be dealt with, and this will make the device unsuitable for practical use.
(B) The latter type of conventional device, namely, the device for coloring figures in the reverse order of priority, is disadvantageous in that the figure of highest priority is sometimes not displayed on the CRT.
In the conventional device of this type, figures are successively colored out according to the data from the figure having a low priority to the figure having a high priority, in the same manner as when coloring a picture in many layers. In this way, a plurality of figures are displayed with overlapping portions.
Therefore, since this device dispenses with the above-described read-modify-write operation, the entire circuit is simplified and the coloring out process can be executed at comparatively high speed.
On the other hand, if data write time is in short supply for one reason or another, it is impossible to write the figure of the highest priority into the memory, and, as a result, the figures having lower priorities alone are displayed on the CRT, the figure of the highest priority being omitted.
(C) Both of these conventional devices which use the bit-map-display technique inconveniently require a very large memory capacity.
Use of the bit-map-memory technique requires a large-capacity bit-map-memory having memory areas which correspond to all the pixels of the CRT.
Specifically, in order to display a desired colored image on the CRT, a memory capacity equivalent to the number of pixels multiplied by the number of color information bits for color display is necessary. Thus, the memory capacity used inconveniently becomes extremely large.